OS X: Can I paste plain text by default?

Here you go: https://www.engadget.com/2009-07-07-tuaw-tip-paste-without-formatting-by-default.html

There's an answer, thanks to the Keyboard & Mouse pane in System Preferences. If you click the Keyboard Shortcuts tab, you can assign "Paste Without Formatting" and/or "Paste and Match Style" to Command + V for all applications. Just click the + button under the list, type in the name of the appropriate menu items, and press ⌘-V in the Keyboard Shortcut box.

One note: Panic designer Neven Mrgan found that using this tip means you won't be able to paste images into iChat using Command + V. Dragging and dropping (or choosing Paste from the Edit menu) still works, though. Update: If you explicitly add ⌘-V in as a shortcut for Paste that's specific to iChat, you can restore the image paste capability without breaking the global shortcut.

PTH Pasteboard gives you this functionality plus a whole lot more. Multiple clipboards, running scripts (python, ruby, bash, etc...) before being pasted, and persistent clipboard history across reboots are some of my favorite features.

I think I bought this utility faster than any other shareware ever, it's a steal at $25.

Another alternative is, CleanBoard which is a lightweight MacOS app that removes any formatting or special characters from your copied text by hitting the copy-shortcut twice. Great if you don't want to remember those complicated keyboard shortcuts.